Allow downloads to bind to a network interface

[thsnkhn]

Problem

Users connected to more than one network cannot choose which interface Harbor uses. Torrent users specifically want to bind traffic to a VPN interface and avoid fallback to another connection.

Goal

Allow direct and torrent downloads to use a selected network interface when the underlying backend supports it.

Acceptance criteria

• Settings provides Automatic and available network-interface choices.
• New direct downloads use the selected interface.
• New torrent downloads pass the selected interface or local address to aria2.
• Harbor does not silently fall back to another interface when a required interface disappears.
• Harbor pauses or fails the affected transfer with a clear message.
• Existing downloads keep predictable behavior when the setting changes.
• The UI identifies VPN and tunnel interfaces when macOS exposes enough information.

Source

Requested in the Harbor 1.7.0 Reddit announcement. Another user described this as a requirement for torrent use.

[InvalidPandaa]

Working on this — PR incoming for the torrent side.

One finding worth recording first: the "New direct downloads use the selected interface" criterion is not reachable with the current direct engine. URLSession/URLSessionConfiguration expose no interface binding on macOS, and kCFStreamPropertyBoundInterfaceIdentifier does not exist there either. The only API that binds a socket to an interface is nw_parameters_require_interface, which would mean rebuilding DownloadCoordinator on NWConnection — resume data, browser handoff and all. That reads like its own change, so the PR covers torrents and states the limitation in the UI rather than implying protection it cannot give. Happy to split direct downloads into a follow-up issue if you would like it tracked separately.

The other six criteria are covered, including VPN and tunnel identification.